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MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION     INTRODUCTION There are two types of MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION     INTRODUCTION There are two types of

MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION INTRODUCTION There are two types of - PDF document

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MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION INTRODUCTION There are two types of - PPT Presentation

The principles on which these devices operate current controlled by an electric field are very similar the primary difference being in the methods by which the control element is made This difference however results in a considerable difference in ID: 24303

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AN211A RFApplicationInformationFreescaleSemiconductorFieldEffectTransistorsinTheoryandPracticeTherearetwotypesoffield-effecttransistors,theJunctionField-EffectTransistor(JFET)andthe“Metal-OxideSemiconductor”Field-EffectTransistor(MOSFET),orInsulated-GateField-EffectTransistor(IGFET).Theprinciplesonwhichthesedevicesoperate(currentcontrolledbyanelectricfield)areverysimilar—theprimarydifferencebeinginthemethodsbywhichthecontrolelementismade.Thisdifference,however,resultsinaconsiderabledifferenceindevicecharacteristicsandnecessitatesvariancesincircuitdesign,whicharediscussedinthisnote. DRAINGATESOURCEN-CHANNELJFETP-CHANNELJFETDRAINGATESOURCEJUNCTIONFIELD-EFFECTTRANSISTOR(JFET)Initssimplestformthejunctionfield-effecttransistorstartswithnothingmorethanabarofdopedsiliconthatbehavesasaresistor(Figure1a).Byconvention,theterminalintowhichcurrentisinjectediscalledthesourceterminal,since,asfarastheFETisconcerned,currentoriginatesfromthisterminal.Theotherterminaliscalledthedrainterminal.Currentflowbetweensourceanddrainisrelatedtothedrain-sourcevoltagebytheresistanceoftheinterveningmaterial.InFigure1b,p-typeregionshavebeendiffusedintothen-typesubstrateofFigure1aleavingann-typechannelbetweenthesourceanddrain.(Acomplementaryp-typedeviceismadebyreversingallofthematerialtypes.)Thesep-typeregionswillbeusedtocontrolthecurrentflowbetweenthesourceandthedrainandarethuscalledgateregions. SOURCE SOURCEDRAIN SOURCEDRAINGATE1 P GATE2 SOURCEDRAIN P GATE2(c) GATE1DEPLETIONZONES DRAIN GATE2 GATE1 D (+)(--)(--) Figure1.DevelopmentofJunctionField-EffectTransistors AN211ARev.0,7/1993FreescaleSemiconductorApplicationNoteNOTE:Thetheoryinthisapplicationnoteisstillapplicable,butsomeoftheproductsreferencedmaybediscontinued. FreescaleSemiconductor,Inc.,1993,2009. llrightsreserved. 2 RFApplicationInformationFreescaleSemiconductorAN211A =--1V =--2V DIPVP (BR)DSS D LOCUS Figure2.DrainCurrentCharacteristicsDuetothedifficultyofdiffusingimpuritiesintobothsidesofasemiconductorwafer,asingleendedgeometryisnormallyusedinsteadofthetwo-sidedstructurediscussedabove.Diffusionforthisgeometry(Figure3)isfromonesideonly.Thesubstrateisofp-typematerialontowhichann-typechannelisgrownepitaxially.Ap-typegateisthendiffusedintothen-typeepitaxialchannel.Contactmetallizationcompletesthestructure.Thesubstrate,whichfunctionsasGate2ofFigure1,isofrelativelylowresistivitymaterialtomaximizegain.Forthesamepurpose,Gate1isofverylowresistivitymaterial,allowingthedepletionregiontospreadmostlyintothen-typechannel.Inmostcasesthegatesareinternallyconnectedtogether.Atetrodedevicecanberealizedbynotmakingthisinternalconnection. DRAINGATESOURCEN-CHANNELMOSFETP-CHANNELMOSFET DRAINGATESOURCE DRAINGATESOURCE DRAINGATESOURCE SUBSTRATESUBSTRATE SUBSTRATESUBSTRATETYPECTYPEB MOSFIELD-EFFECTTRANSISTORS(MOSFET)Themetal-oxide-semiconductor(MOSFET)operateswithaslightlydifferentcontrolmechanismthantheJFET.Figure4showsthedevelopment.Thesubstratemaybehighresistivityp-typematerial,asforthe2N4351.Thistimetwoseparatelow-resistivityn-typeregions(sourceanddrain)arediffusedintothesubstrateasshowninFigure4b.Next,thesurfaceofthestructureiscoveredwithaninsulatingoxidelayerandanitridelayer.TheoxidelayerservesasaprotectivecoatingfortheFETsurfaceandtoinsulatethechannelfromthegate.Howevertheoxideissubjecttocontaminationbysodiumionswhicharefoundinvaryingquantitiesinallenvironments.Suchcontaminationresultsinlongterminstabilityandchangesindevicecharacteristics.Siliconnitrideisimpervioustosodiumionsandthusisusedtoshieldtheoxidelayerfromcontamination.Holesarecutintotheoxideandnitridelayersallowingmetalliccontacttothesourceanddrain.Then,thegatemetalareaisoverlaidontheinsulation,coveringtheentirechannelregionand,simultaneously,metalcontactstothedrainandsourcearemadeasshowninFigure4d.Thecontacttothemetalareacoveringthechannelisthegateterminal.Notethatthereisnophysicalpenetrationofthemetalthroughtheoxideandnitrideintothesubstrate.Sincethedrainandsourceareisolatedbythesubstrate,anydrain-to-sourcecurrentintheabsenceofgatevoltageisextremelylowbecausethestructureisanalogoustotwodiodesconnectedbacktoback.Themetalareaofthegateformsacapacitorwiththeinsulatinglayersandthesemiconductorchannel.Themetalareaisthetopplate;thesubstratematerialandchannelarethebottomplate.ForthestructureofFigure4,considerapositivegatepotential(seeFigure5).Positivechargesatthemetalsideofthemetal-oxidecapacitorinduceacorrespondingnegativechargeatthesemiconductorside.Asthepositivechargeatthegateisincreased,thenegativecharge“induced”inthesemiconductorincreasesuntiltheregionbeneaththeoxideeffectivelybecomesann-typesemiconductorregion,andcurrentcanflowbetweendrainandsourcethroughthe“induced”channel.Inotherwords,draincurrentflowis“enhanced”bythegatepotential.Thusdraincurrentflowcanbemodulatedbythegatevoltage;i.e.thechannelresistanceisdirectlyrelatedtothegatevoltage.Then-channelstructuremaybechangedtoap-channeldevicebyreversingthematerialtypes. SOURCECHANNELDRAINSOURCEGATE LP (SUBSTRATE) CHANNELLENGTH Figure3.JunctionFETwithSingle-EndedGeometry DRAINSOURCE NNNN(SUBSTRATE)(SUBSTRATE)(c)(d)OXIDE SILICONNITRATE(SUBSTRATE)(SUBSTRATE) SGD METALFigure4.DevelopmentofEnhancement-ModeN-ChannelMOSFET AN211A RFApplicationInformationFreescaleSemiconductor +++++++++ DRAINGATE(+)SOURCEP(SUBSTRATE) ------------------ ∓∓∓∓∓∓∓∓∓ INDUCEDCHANNELALUMINUM Figure5.ChannelEnhancement.ApplicationofPositiveGateVoltageCausesRedistributionofMinorityCarriersintheSubstrateandResultsintheFormationofaConductiveChannelBetweenSourceandDrainAnequivalentcircuitfortheMOSFETisshowninFigure6.Here,Cisthedistributedgate-to-channelcapacitancerepresentingthenitride-oxidecapacitance.Cisthegate-sourcecapacitanceofthemetalgateareaoverlappingthesource,whileCisthegate-draincapacitanceofthemetalgateareaoverlappingthedrain.andCarejunctioncapacitancesfromdraintosubstrateandsourcetosubstrate.Yisthetransadmittancebetweendraincurrentandgate-sourcevoltage.ThemodulatedchannelresistanceisrandRarethebulkresistancesofthedrainandsource.TheinputresistanceoftheMOSFETisexceptionallyhighbecausethegatebehavesasacapacitorwithverylowleakage(r).Theoutputimpedanceisafunctionofr(whichisrelatedtothegatevoltage)andthedrainandsourcebulkresistances(RandRToturntheMOSFET“on”,thegate-channelcapacitance,,andtheMillercapacitance,C,mustbecharged.Inturning“on”,thedrain-substratecapacitance,C,mustbedischarged.Theresistanceofthesubstratedeterminesthepeakdischargecurrentforthiscapacitance.TheFETjustdescribediscalledanenhancement-typeMOSFET.Adepletion-typeMOSFETcanbemadeinthefollowingmanner:StartingwiththebasicstructureofFigure4,amoderateresistivityn-channelisdiffusedbetweenthesourceanddrainsothatdraincurrentcanflowwhenthegatepotentialisatzerovolts(Figure7).Inthismanner,theMOSFETcanbemadetoexhibitdepletioncharacteristics.Forpositivegatevoltages,thestructureenhancesinthesamemannerasthedeviceofFigure4.Withnegativegatevoltage,theenhancementprocessisreversedandthechannelbeginstodepleteofcarriersasseeninFigure8.AswiththeJFET,drain-currentflowdepletesthechannelareanearestthedrainfirst.ThestructureofFigure7,therefore,isbothadepletion-modeandanenhancement-modedevice.MODESOFOPERATIONTherearetwobasicmodesofoperationofFET’s—depletionandenhancement.Depletionmode,aspreviouslymentioned,referstothedecreaseofcarriersinthechannelduetovariationingatevoltage.Enhancementmodereferstotheincreaseofcarriersinthechannelduetoapplicationofgatevoltage.AthirdtypeofFETthatcanoperateinboththedepletionandtheenhancementmodeshasalsobeendescribed.ThebasicdifferencesbetweenthesemodescanmosteasilybeunderstoodbyexaminingthetransfercharacteristicsofFigure9.Thedepletion-modedevicehasconsiderabledrain-currentflowforzerogatevoltage.Draincurrentisreducedbyapplyingareversevoltagetothegateterminal.Thedepletion-typeFETisnotcharacterizedwithforwardgatevoltage.Thedepletion/enhancementmodetypedevicealsohasconsiderabledraincurrentwithzerogatevoltage.Thistypedeviceisdefinedintheforwardregionandmayhaveusableforwardcharacteristicsforquitelargegatevoltages.NoticethatforthejunctionFET,draincurrentmaybeenhancedbyforwardgatevoltageonlyuntilthegate-sourcep-njunctionbecomesforwardbiased.ThethirdtypeofFEToperatesonlyintheenhancementmode.ThisFEThasextremelylowdraincurrentflowforzerogate-sourcevoltage.Draincurrentconductionoccursforagreaterthansomethresholdvalue,VGS(th).Forgatevoltagesgreaterthanthethreshold,thetransfercharacteristicsaresimilartothedepletion/enhancementmodeFET. DRAINGATESOURCEPSUBSTRATE S S(sub) Figure6.EquivalentCircuitofEnhancement-ModeMOSFET DRAINGATESOURCEP(SUBSTRATE) N+ ALUMINUM DIFFUSEDCHANNEL Figure7.DepletionModeMOSFETStructure.ThisTypeofDeviceMayBeDesignedtoOperateinBoththeEnhancementandDepletionModes 4 RFApplicationInformationFreescaleSemiconductorAN211A -------------------------- DRAINGATESOURCEP(SUBSTRATE) N+ +++++++++ (--)Figure8.ChannelDepletionPhenomenon.ApplicationofNegativeGateVoltageCausesRedistributionofMinorityCarriersinDiffusedChannelandReducesEffectiveChannelThickness.ThisResultsinIncreasedChannelResistance.ELECTRICALCHARACTERISTICSBecausethebasicmodeofoperationforfield-effectdevicesdiffersgreatlyfromthatofconventionaljunctiontransistors,theterminologyandspecificationsarenecessarilydifferent.AnunderstandingofFETterminologyandcharacteristicsarenecessarytoevaluatetheircomparativemeritsfromdata-sheetspecifications.StaticCharacteristicsStaticcharacteristicsdefinetheoperationofanactivedeviceundertheinfluenceofapplieddcoperatingconditions.Ofprimaryinterestarethosespecificationsthatindicatetheeffectofacontrolsignalontheoutputcurrent.TheVtransfercharacteristicscurvesareillustratedinFigure9forthethreetypesofFETs.Figure10liststhedata-sheetspecificationsnormallyemployedtodescribethesecurves,aswellasthetestcircuitsthatyieldtheindicatedspecifications.Ofadditionalinterestisthespecialcaseoftetrode-connecteddevicesinwhichthetwogatesareseparatelyaccessiblefortheapplicationofacontrolsignal.Thepertinentspecificationsforajunctiontetrodearethosewhichdefinedrain-currentcutoffwhenoneofthegatesisconnectedtothesourceandthebiasvoltageisappliedtothesecondgate.TheseareusuallyspecifiedasVG1S(off)Gate1—sourcecutoffvoltage(withGate2connectedtosource),andVG2S(off),Gate2—sourcecutoffvoltage(withGate1connectedtosource).Thegatevoltagerequiredfordraincurrentcutoffwithoneofthegatesconnectedtothesourceisalwayshigherthanthatforthetriode-connectedcasewherebothgatesaretiedtogether. (--)(--) (+)(+) ) =4V =1V/STEP =1V/STEP =1V/STEP --1V --2V =2V --1V --2V N-CHANNEL DEPLETIONENHANCEMENT DEPLETIONENHANCEMENT G S V G S ( t h ) ID DEPLETIONENHANCEMENTENHANCEMENTONLYDEPLETIONONLYDEPLETIONANDENHANCEMENTMODE N-CHANNELJUNCTIONFETN-CHANNELMOS2N3797N-CHANNELMOS2N4351 =2V/DIV =2V/DIV =2V/DIV=1mA/DIV=1mA/DIV=1mA/DIV NOTDEFINEDINTHISAREAFigure9.TransferCharacteristicsandAssociatedScopeTracesfortheThreeFETTypes AN211A RFApplicationInformationFreescaleSemiconductor DRAINGATE*SOURCE TESTCIRCUITFORVANDVGS(off) DRAINGATE*SOURCE TESTCIRCUITFORIDSS *GATESINTERNALLYCONNECTED†ADJUSTFORDESIREDIDRAIN SOURCE DSS0.1IDSS D(off)GS(off)GATEVOLTAGE DSSD(on)GS(off)GATEVOLTAGE DEPLETIONMODEDEPLETIONMODE CHARACTERISTICDESCRIPTIONDSS=0,(BR)DSSGS(off)=0.001IDSS(BR)DSS =0.1IDSS(BR)DSSZero-gate-voltagedraincurrent.Representsmaximumdraincurrent.GatevoltagenecessarytoreducetosomespecifiednegligiblevalueattherecommendedVi.e.cutoff.GatevoltageforaspecifiedvalueofbetweenIDSSandIatcutoff--normally0.1IDSS DSSD(on)GS(th)GATEVOLTAGE ENHANCEMENTMODEENHANCEMENTMODE 0.1I CHARACTERISTICDESCRIPTIOND(on)�0,(BR)DSSGS(off)=0.001IDSSAnarbitrarycurrentvalue(usuallynearmaxratedcurrent)thatlocatesapointintheenhancementoperationmode.Zero-gate-voltagedraincurrent.VoltagenecessarytoreduceIsomespecifiednegligiblevalueattherecommendedV,i.e.cutoff.TESTCIRCUITFORIDEPLETION/ENHANCEMENTMODEMOSFETsENHANCEMENTMODEMOSFETsDSS=0,(BR)DSS CHARACTERISTICDESCRIPTIOND(on)�0,(BR)DSSGS(th)=0.001D(on)orlessGate-sourcevoltageforaspecifieddraincurrentof0.1IGatecutofforturn-onvoltage.@0.1ID(on) Anarbitrarycurrentvalue(usuallynearmaxratedcurrent)thatlocatesapointintheenhancementoperationmode.Leakagedraincurrent. DSS=0,(BR)DSSDEPLETIONMODEJFETsTESTCIRCUITSAMEASFORITESTCIRCUITSAMEASFORDEPLETION/ENHANCEMENTGS(th)TESTCIRCUITSAMEASFORVGS(off)FORJFETEXCEPTREVERSEVBATTERYPOLARITYDSSTESTCIRCUITSAMEASFORJFET†ADJUSTFORDESIREDINORMALLYNEARMAX-RATEDITESTCIRCUITFORIDSSANDVGS(off)GATEFigure10.StaticCharacteristicsfortheThreeFETTypesAreDefinedbytheAboveCurves,Tables,andTestCircuitsReach-throughvoltageisanotherspecificationuniquelyapplicabletotetrode-connecteddevices.Thisdefinestheamountofdifferencevoltagethatmaybeappliedtothetwogatesbeforethedepletionregionofonespreadsintothejunctionoftheother—causinganincreaseingatecurrenttosomesmallspecifiedvalue.Obviously,reach-throughisanundesirableconditionsinceitcausesadecreaseininputresistanceasaresultofanincreasedgatecurrent,andlargeamountsofreach-throughcurrentcandestroytheFET.GateLeakageCurrentOfinteresttocircuitdesignersistheinputresistanceofanactivecomponent.ForFETs,thischaracteristicisspecifiedintheformofIGSS—thereverse-biasgate-to-sourcecurrentwiththedrainshortedtothesource(Figure11).Asmightbeexpected,becausetheleakagecurrentacrossareverse-biasedp-njunction(inthecaseofaJFET)andacrossacapacitor(inthecaseofaMOSFET)isverysmall,theinputresistanceisextremelyhigh.Atatemperatureof25C,theJFETinputresistanceishundredsofmegohmswhilethatofaMOSFETisevengreater.Forjunctiondevices,however,inputresistancemaydecreasebyseveralordersofmagnitudeastemperatureisraisedtoC.Suchdevices,therefore,havegate-leakagecurrentspecifiedattwotemperatures.Insulated-gateFETsarenotdrasticallyaffectedbytemperature,andtheirinputresistanceremainsextremelyhighevenatelevatedtemperatures. 6 RFApplicationInformationFreescaleSemiconductorAN211A DRAINGATESOURCE nA/GS(off)(BR)DGO Figure11.TestCircuitforLeakageCurrentGateleakagecurrentmayalsobespecifiedasI(leakagebetweengateanddrainwiththesourceopen),orasIGSO(leakagebetweengateandsourcewiththedrainopen).Theseusuallyresultinlowervaluesofleakagecurrentanddonotrepresentworst-caseconditions.TheIGSSspecification,therefore,isusuallypreferredbytheuser.VoltageBreakdownAvarietyofspecificationscanbeusedtoindicatethemaximumvoltagethatmaybeappliedtovariouselementsofaFET.Amongthoseincommonusearethefollowing:(BR)GSS=Gate-to-sourcebreakdownvoltage=Drain-to-gatebreakdownvoltage=Drain-to-sourcebreakdownvoltage(normallyusedonlyforMOSFETs)Inaddition,theremayberatingsandspecificationsindicatingthemaximumvoltagesthatmaybeappliedbetweentheindividualgatesandthedrainandsource(fortetrodeconnecteddevices).Obviously,notallofthesespecificationsarefoundoneverydatasheetsincesomeofthemprovidethesameinformationinsomewhatdifferentform.Byunderstandingthevariousbreakdownmechanisms,however,thereadershouldbeabletointerprettheintentofeachspecificationandrating.Forexample:InjunctionFETs,themaximumvoltagethatmaybeappliedbetweenanytwoterminalsisthelowestvoltagethatwillleadtobreakdownoravalancheofthegatejunction.TomeasureV(BR)GSS(Figure12a),anincreasinglyhigherreversevoltageisappliedbetweenthegateandthesource.Junctionbreakdownisindicatedbyanincreaseingatecurrent(beyondIGSS)whichsignalsthebeginningofavalanche.SomereflectionwillrevealthatforjunctionFETs,thespecificationreallyprovidesthesameinformationasV(BR)GSS.Forthismeasurement,anincreasingvoltageisappliedbetweendrainandgate.Whenthisappliedvoltagebecomeshighenough,thedrain-gatejunctionwillgointoavalanche,indicatedeitherbyasignificantincreaseindraincurrentorbyanincreaseingatecurrent(beyondI).ForbothVandV(BR)GSSspecifications,breakdownshouldnormallyoccuratthesamevoltagevalue.FromFigure2itisseenthatavalancheoccursatalowervalueofVwhenthegateisreversebiasedthanforthezero-biascondition.Thisiscausedbythefactthatthereverse-biasgatevoltageaddstothedrainvoltage,therebyincreasingtheeffectivevoltageacrossthejunction.Themaximumamountofdrain-sourcevoltagethatmaybeappliedVis,therefore,equaltoV,whichindicatesavalanchewithreversebiasgatevoltageapplied.ForMOSFETs,thebreakdownmechanismissomewhatdifferent.Consider,forexample,theenhancement-modestructureofFigure5.Here,thegateiscompletelyinsulatedfromthedrain,source,andchannelbyanoxide-nitridelayer.Thebreakdownvoltagebetweenthegateandanyoftheotherelements,therefore,isdependentonthethicknessandpurityofthisinsulatinglayer,andrepresentsthevoltagethatwillphysicallypuncturethelayer.Consequently,thevoltagemustbespecifiedseparately.Thedrain-to-sourcebreakdownisadifferentmatter.Forenhancementmodedevices,withthegateconnectedtothesource(thecutoffcondition)andthesubstratefloating,thereisnoeffectivechannelbetweendrainandsourceandtheapplieddrain-sourcevoltageappearsacrosstwoopposedseriesdiodes,representedbythesource-to-substrateandsubstrate-to-drainjunctions.Draincurrentremainsataverylowlevel(picoamperes)asdrainvoltageisincreaseduntilthedrainvoltagereachesavaluethatcausesreverse(avalanche)breakdownofthediodes.Thisparticularcondition,representedbyV,isindicatedbyanincreaseinIabovetheIlevel,asshowninFigure12b.Fordepletion/enhancementmodedevices,theVsymbolissometimesreplacedbyV.Notethattheprincipaldifferencebetweenthetwosymbolsisthereplacementofthelastsubscriptswiththesubscriptx.Whereasthesnormallyindicatesthatthegateisshortedtothesource,thexindicatesthatthegateisbiasedtocutofforbeyond.Toachievecutoffinthesedevices,adepletingbiasvoltagemustbeappliedtothegate,Figure12b.AnimportantstaticcharacteristicforswitchingFETsisthe“on”drain-sourcevoltageV.ThischaracteristicfortheMOSFETsisafunctionofV,andresemblestheVCE(sat)versusIcharacteristicsofjunctiontransistors.Thecurveforthesecharacteristicscanbeusedasadesignguidetodeterminetheminimumgatevoltagenecessarytoachieveaspecifiedoutputlogiclevel. Figure12a.VTestCircuit TYPECTYPEB A V A V Figure12b.V(BR)DSSandTestCircuit(UsuallyUsedforMOSFETsOnly). AN211A RFApplicationInformationFreescaleSemiconductorDynamicCharacteristicsUnlikethestaticcharacteristics,thedynamiccharacteristicsoffield-effecttransistorsapplyequallytoallFETs.Theconditionsandpresentationofthedynamiccharacteristics,however,dependlargelyupontheintendedapplication.Forexample,thefollowingtableindicatesthedynamiccharacteristicsneededtoadequatelydescribeaFETforvariousapplications. RF-IF (1kHz) (1kHz) rss rss rss rss (1kHz) (sub) (sub) )(HF) ds(on) ds(on) )(HF) r Theforwardtransadmittanceisakeydynamiccharacteristicforfield-effecttransistors.Itservesasabasicdesignparameterinaudioandrfcircuitsandisawidelyacceptedfigureofmeritfordevices.Becausefield-effecttransistorshavemanycharacteristicssimilartothoseofvacuumtubes,andbecausemanyengineersstillaremorecomfortablewithtubeparameters,thesymbolgusedfortubetransconductanceisoftenspecifiedinsteadofy.Tofurtherconfusethings,the“g”schoolalsousesavarietyofsubscripts.InadditiontogsomedatasheetsshowgwhileothersevenshowgRegardlessofthesymbolused,ydefinestherelationbetweenaninputsignalvoltageandanoutputsignalcurrent: Theunitisthemho—currentdividedbyvoltage.Figure13isatypicalytestcircuitforajunctionFET.Asacharacteristicofallfield-effectdevices,yspecifiedat1kHzwithaVthesameasthatforwhichorIischaracterized.Sinceyhasbothrealandimaginarycomponents,butisdominatedbytherealcomponentatlowfrequency,the1kHzcharacteristicisgivenasanabsolutemagnitudeandindicatedasItisinterestingtonotethatyvariesconsiderablywithduetononlinearityintheI--Vcharacteristics.Thisvariation,foratypicaln-channel,JFETisillustratedinFigure14.Obviously,theoperatingpointmustbecarefullyselectedtoprovidethedesiredyandsignalswing.Fortetrode-connectedFETs,threeymeasurementsareusuallyspecifiedondata-sheettables.Oneofthese,withthetwogatestiedtogether,providesayvaluefortheconditionwhereasignalisappliedtobothgatessimultaneously;theothersprovidetheyforthetwogatesindividually.Generally,withthetwogatestiedtogether,yishigherandmoregainmayberealizedinagivencircuit.Becauseoftheincreasedcapacitance,however,gain-bandwidthproductismuchlower.Forrffield-effecttransistors,anadditionalvalueofysometimesspecifiedatornearthehighestfrequencyofoperation.ThisvalueshouldalsobemeasuredatthesamevoltageconditionsasthoseusedforIorI.Becauseoftheimportanceoftheimaginarycomponentatradiofrequencies,thehighfrequencyyspecificationshouldbeacomplexrepresentation,andshouldbegiveneitherinthespecificationstableorbymeansofcurvesshowingtypicalvariations,asinFigure15fortheMPF102JFET.Therealportionofthishigh-frequencyy,Re(y)orGisusuallyconsideredasignificantfigureofmerit.AnotherFETparameterthatoffersadirectvacuumtubeanalogyisy,theoutputadmittance: Inthiscase,theanalogoustubeparameterisr—i.e.,=1/r.Fordepletionmodedevices,yismeasuredwithgateandsourcegrounded(seeFigure16).Forenhancementmodeunits,itismeasuredatsomespecifiedVpermitssubstantialdrain-currentflow.Aswithy,manyexpressionsareusedfory.Inadditiontotheobviousparallelssuchasy,andg,itisalsosometimesspecifiedasr,wherer=1/y V VTVMACVTVM fsl TYPICALLY1MOFSUCHVALUEASTOCAUSENEGLIGIBLEDCDROPATIDSSBYPASSFigure13.TypicalyTestCircuit 0.10.20.51.02.05.010 2N4222,A 2N4221,A 2N4220,A,FORWARDTRANSFERADMITTANCE(mhos),DRAINCURRENT(mA) =15V=25f=1.0kHzFigure14.ForwardTransferAdmittanceversusDrainCurrentforTypicalJFETs 8 RFApplicationInformationFreescaleSemiconductorAN211A --b =15Vdcf,FREQUENCY(MHz)FORWARDTRANSFERADMITTANCE(mmhos)Figure15.ForwardTransferAdmittanceversusFrequency VTVMDRSGIYV SENSESACDRAINCURRENTOFSUCHVALUEASTOCAUSENEGLIGIBLEDCDROP. BYPASS Figure16.yMeasurementCircuitforDepletionFETsVoltagesandfrequenciesformeasuringyshouldbeexactlythesameasthoseformeasuringy.Likey,itisacomplexnumberandshouldbespecifiedasamagnitudeat1kHzandincomplexformathighfrequencies.Closelyrelatedtoyandyistheamplificationfac-tor, Theamplificationfactordoesnotappearonthefield-effecttransistorregistrationformatbutcanbecalculatedasyFormostsmall-signalapplications,haslittlecircuitsignificance.Itdoes,however,serveasageneralindicationofthequalityofthefield-effectmanufacturingprocess.Thecommon-source-circuitinputcapacitance,Cisstakestheplaceofyinlow-frequencyfield-effecttransistors.Thisisbecauseyisentirelycapacitiveatlowfrequencies.issisconvenientlymeasuredinthecircuitofFigure17forthetetrodeJFET.Aswithy,twomeasurementsarenecessaryfortetrode-connecteddevices.Atveryhighfrequencies,therealcomponentofybecomesimportantsothatrffield-effecttransistorsshouldhaveyspecifiedasacomplexnumberatthesameconditionsasotherhigh-frequencyparameters.Fortetrode-connectedrfFETs,readingofbothGate2tosourceandGate1tiedtoGate2arenecessary.InswitchingapplicationsCissisofmajorimportancesincealargevoltageswingatthegatemustappearacrossCissThus,Cissmustbechargedbytheinputvoltagebeforeturn-oneffectivelybegins.rssReversetransferadmittance(y)doesnotappearonFETdatasheets.InsteadC,thereversetransfercapacitance,isspecifiedatlowfrequency.Sinceyforafield-effecttransistorremainsalmostcompletelycapacitiveandrelativelyconstantovertheentireusableFETfrequencyspectrum,thelow-frequencycapacitanceisanadequatespecification.CismeasuredbythecircuitofFigure18.FortetrodeFETs,valuesshouldbespecifiedforGate1andforbothgatestiedtogether.Again,forswitchingapplicationsCisacriticalcharacteristic.SimilartotheCofajunctiontransistor,Cmustbechargedanddischargedduringtheswitchinginterval.Forachopperapplication,Cisthefeedthroughcapacitanceforthechopperdrive.d(sub)FortheMOSFET,thedrain-substratejunctioncapacitancebecomesanimportantcharacteristicaffectingtheswitchingbehavior.Cappearsinparallelwiththeloadinaswitchingcircuitandmustbechargedanddischargedbetweenthetwologiclevelsduringtheswitchinginterval. DGIC BYPASS CAPACI-TANCEFigure17.CMeasurementCircuit DGIC GUARD g CAPACI-TANCE (a)V=--VTYPICALLY1MACSOURCETOGUARDSIGNAL.Figure18.RecommendedCrssTestCircuit AN211A RFApplicationInformationFreescaleSemiconductorNoiseFigure(NF)Likeallotheractivecomponents,field-effecttransistorsgenerateacertainamountofnoise.Thenoisefigureforfield-effecttransistorsisnormallyspecifiedonthedatasheetas“spotnoise”,referringtothenoiseataparticularfrequency.Thenoisefigurewillvarywithfrequencyandalsowiththeresistanceattheinputofthedevice.TypicalgraphsofsuchvariationsareillustratedinFigure19forthe2N5458.Fromgraphsofthiskindthedesignercananticipatethenoiselevelinherentinhisdesign.ds(on)Channelresistancedescribesthebulkresistanceofthechannelinserieswiththedrainandsource.Fromanapplicationsstandpoint,itisimportantprimarilyforswitchingandchoppercircuitssinceitaffectstheswitchingspeedanddeterminestheoutputlevel.TocompletetheconfusionofmultiplesymbolsforFETparameters,channelresistanceissometimesindicatedasrandalsoasrandr.Ineithercase,however,itismeasured,forJFETs,bytyingthegatestothesource,settingallterminalsequalto0Vdc,andapplyinganacvoltagefromdraintosource(seeFigure20).Themagnitudeoftheacvoltageshouldbekeptlowsothattherewillbenopinchoffinthechannel.Insulated-gateFETsmaybemeasuredwithdcgatebiasintheenhancement =15VDSS=250.51.0251020501001kHz10kHzNF,NOISEFIGURE(dB) =1M f=1kHzf,FREQUENCY(Hz),SOURCERESISTANCE(MEGOHMS)——Figure19.TypicalVariationsofFETNoiseFigurewithFrequencyandSourceResistance ACAM- i Figure20.CircuitforMeasuringJFETChannelResistanceAPPLICATIONSDeviceSelectionObviously,differentapplicationscallforspecialemphasisonspecificcharacteristicssothatasimplefigureofmeritthatcomparesdevicesforallpotentialuseswouldbehardtoformulate.Nevertheless,anattempttopinpointthecharacteristicsthataremostsignificantforvariousapplicationshasbeenmade*topermitarapid,first-orderevaluationofcompetitivedevices.ThemostimportantsingleFETparameter,onethatappliesforanyamplifierapplication,isy.Thisparameter,oroneofitsmanyvariations,isspecifiedonmostdatasheets,yetsomeevaluationisrequiredtocomeupwithareasonablecomparison.Forexample,inthetableofelectricalcharacteristicsonmostJFETdatasheets,yspecifiedatI=0)where,forJFETsdevices,yismaximum.ThisisillustratedinFigure14,wheretypicalvariationsofyasafunctionofIareplotted.Forsomesmall-signalapplications,theI=0)pointcanactuallybeusedasadcoperatingpointbecausesmall-signalexcursionsintotheforwardbiasregionwillnotactuallycausethegate-sourcejunctiontobecomeforward-biased.However,inmostpracticaluses,somebiasisnecessarytoallowfortheanticipatedsignalswing;anditmustberecognizedtheygoesdownasthebiasisincreased.Itisseen,also,thatmaximumyincreasesasIincreasessothat,wheremaximumyisimportant,adevicewithahighIspecificationisnormallydesirable.Ontheotherhand,wherepowerdissipationisafactortobeconsidered,thefigureofmerityGS(off)beenproposed.ThistermfactorsinnotonlyI,whichshouldbelowifpowerdissipationistobelow,butalsoGS(off),whichindicatesmaximuminputvoltageswing.SincethesignalpeaksarerepresentedbyVGS(off)andV=0,thelowerVGS(off),thehigherthefigureofmerit.And,foramplifierapplicationsrequiringalargesignalswing,(BR)GSSGS(off)(assumingthatVGS(off)isthe“pinch-off”voltage)isasatisfactorymeritfigurebecauseitindicatestheratioofmaximumandminimumdrainvoltages.Forhigh-frequencycircuits,theinputcapacitance(CissandtheMiller-effectcapacitance(C)becomeimportant,soyiss)indicatesarelativemeasureofdeviceperformance.Forswitchingandchoppercircuits,afigureofmeritisnotoftenuseful.HerethemagnitudesofCiss)andrareofprimaryinterest.CircuitsThetypesofcircuitsthatcanutilizeFETsarepracticallyunlimited.Infact,manycircuitsdesignedtoutilizesmall--signalpentodetubescanutilizeFETswithonlyminormodifications.Forexample,thecircuitinFigure21showsatypicalrfstageforabroadcast-bandautoradio.Inthiscircuit,aMPF102n-channelJFEThasreplacedthe12BL6pentodenormallyemployed.Thespecificationsforthetwodevices,includingtheAGCcharacteristics,aresimilarenoughtoperformadequatelyinthecircuitofFigure21. *Christiansen,Donald,“Semiconductors:TheNewFiguresofMerit,”EEE,October,1965. 10 RFApplicationInformationFreescaleSemiconductorAN211A DGI TOMIXER +12V MPF102 TUNING Figure21.RFStageofBroadcastAutoRadioInanaudioapplication,afield-effecttransistorsuchasthe2N5460canbecombinedwithahighvoltagebipolartransistortomakeasimpleline-operatedphonographamplifiersuchasthatshowninFigure22.Theceramicpickupisconnectedthroughapotentiometervolumecontroltothefield-effecttransistor.Collectorcurrentofthetransistor,inturn,issetbythepotentiometerinthesourceoftheFET.Withtheproperbipolaroutputtransistor,thecircuitcanbedrivendirectlyfromtherectifiedlinevoltage,whilethelowvoltagefortheFETcanbederivedfromavoltagedividerinthepowersupplyline. DG MFE4007VOLUME TONE ADJUST--30V--120V Figure22.LineOperatedPhonoAmplifiersFigure23showsthreebasicchoppercircuits.Theadvantageofthemorecomplexseries-shuntcircuit(Figure24c)isthatitbalancesouttheleakagecurrentsoftheFETsinordertoreducevoltageerrorandisusedtoattainhighchoppingfrequencies.Fromanapplicationsstandpoint,theFETcircuitissuperiortoajunctiontransistorcircuitinthatthereisnooffsetvoltagewiththeFETturnedon.Ontheminusside,however,thefield-effect-transistorchoppergenerallyhasahigherseriesresistance(r)thanthejunctiontransistor.AsnewerandbetterFETsareintroducedandasalargernumberofdesignerslearntousethem,therangeofapplicationsofFETsshouldbroadenconsiderably.Withitshighinputimpedance,thefield-effecttransistorwillplayanimportantroleininputcircuitryforinstrumentationandaudioapplicationswherelow-impedancejunctiontransistorshavegenerallybeenleastsuccessful. (a)SERIESCHOPPERCIRCUIT MFE2012 SRLesec MFE2012(b)SHUNTCHOPPER s MFE2012 MFE2012 (c)SERIES-SHUNTCHOPPER Figure23.FETChopperCircuits AN211A RFApplicationInformationFreescaleSemiconductorInformationinthisdocumentisprovidedsolelytoenablesystemandsoftwareimplementerstouseFreescaleproducts.Therearenoexpressorimpliedcopyrightlicensesgrantedhereundertodesignorfabricateanyintegratedcircuitsbasedontheinformationinthisdocument.Freescalereservestherighttomakechangeswithoutfurthernoticetoanyproductsherein.Freescalemakesnowarranty,representation,orguaranteeregardingthesuitabilityofitsproductsforanyparticularpurpose,nordoesFreescaleassumeanyliabilityarisingoutoftheapplicationoruseofanyproductorcircuit,andspecificallydisclaimsanyandallliability,includingwithoutlimitationconsequentialorincidentaldamages.“Typical”parametersthatmaybeprovidedinFreescaledatasheetsand/orspecificationscananddovaryindifferentapplications,andactualperformancemayvaryovertime.Alloperatingparameters,including“typicals,”mustbevalidatedforeachcustomerapplicationbycustomer’stechnicalexperts.Freescaledoesnotconveyanylicenseunderitspatentrightsnortherightsofothers.Freescalesellsproductspursuanttostandardtermsandconditionsofsale,whichcanbefoundatthefollowingaddress:freescale.com/SalesTermsandConditions.FreescaleandtheFreescalelogoaretrademarksofFreescaleSemiconductor,Inc.,Reg.U.S.Pat.&Tm.Off.Allotherproductorservicenamesarethepropertyoftheirrespectiveowners.1993,2009FreescaleSemiconductor,Inc. 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